1. Field of the Invention
This invention relates to a silicon carbide light emitting diode having a p-n junction, and more particularly, it relates to a silicon carbide light emitting diode which can attain stable emission of visible light with a short wavelength corresponding to a color in the range of green to purple or stable emission of near ultraviolet light, with high luminous efficiency. This invention also relates to a method for producing such a silicon carbide light emitting diode.
2. Description of the Prior Art
Since light emitting diodes are a small luminescent source which dissipates a significantly little amount of power and can provide stable light emission of high brightness, they are widely used as a display element in a variety of display units. They are also used as a light source for reading recorded data in a variety of data processing units. In particular, light emitting diodes capable of emitting light with a longer wavelength corresponding to a color in the range of from red to green have been widely put into practical use. On the other hand, light emitting diodes capable of emitting visible light of a shorter wavelength corresponding to a color in the range of from blue to purple are now being developed, but they have not yet attained light emission with sufficient brightness for practical use.
In general, the color of light emitted from a light emitting diode depends on the semiconductor material used therefor. Semiconductor materials to be used for light emitting diodes capable of emitting visible light with a short wavelength are limited to silicon carbide (SiC) which is a IV--IV group compound semiconductor, gallium nitride (GaN) which is a III-V group compound semiconductor, and zinc sulfide (ZnS) and zinc selenide (ZnSe) which are II-VI group compound semiconductors. With the use of these semiconductor materials, extensive research has been conducted in order to develop light emitting diodes capable of emitting visible light with a short wavelength. However, mass production of such light emitting diodes with brightness and stability sufficient for practical use has not yet been realized.
For the structure of light emitting diodes, a p-n junction structure is most suited because electrons and holes as carriers can be injected in a light emitting region with high efficiency. However, among the above-mentioned semiconductor materials for light emitting diodes capable of emitting visible light with a short wavelength, it is impossible to use any of the GaN, ZnS, and ZnSe semiconductors for the production of p-n junction light emitting diodes. This is because it is difficult to obtain p-type crystals from these semiconductor materials, or, even if these crystals are obtained, they have high resistance and are very unstable. Therefore, a metal-insulator-semiconductor (MIS) structure using a thin insulating layer or high resistive layer as an insulator has been employed instead of a p-n junction structure. However, light emitting diodes with such an MIS structure have the disadvantages of having non-uniform device characteristics and of providing unstable light emission.
On the other hand, it is possible to use silicon carbide as a material for light emitting diodes of the p-n junction type, because both p-type crystals and n-type crystals can readily be obtained. Many reports have already been made on blue light emitting diodes of the p-n junction type using silicon carbide grown by liquid phase epitaxy (LPE) (see, e.g., M. Ikeda, T. Hayakawa, S. Yamagiwa, H. Matsunami, and T. Tanaka, Journal of Applied Physics, Vol. 50, No. 12, pp. 8215-8225, 1979).
However, conventional blue light emitting diodes produced by liquid phase epitaxy, as described above, can only provide light emission with a brightness of 15 mcd or lower under the operation condition of 20 mA. The principal cause for this low brightness is considered to be as follows. The growth temperature is as high as 1700.degree. C. to 1800.degree. C., so that the crystal growth of silicon carbide takes place in active molten silicon, thereby making it difficult to accurately control the crystal growth, and also having a great possibility that unnecessary impurities will enter the growing crystals. Furthermore, there is the disadvantage that the use of liquid phase expitaxy cannot allow the mass production of blue light emitting diodes.
The inventors have recently devised a method for producing light emitting diodes of the p-n junction type, in which the crystal growth of silicon carbide is controlled with high accuracy by chemical vapor deposition (CVD), thereby allowing the mass production of p-n junction light emitting diodes which can attain stable emission of intense visible light with a short wavelength corresponding to a color in the range of from blue to purple.
However, the above conventional light emitting diodes produced by liquid phase epitaxy or chemical vapor deposition provide an emission spectrum having a wide peak width at half height, so that the light emitted from the diodes has poor monochromaticity. FIG. 8 shows a typical emission spectrum of a blue light emitting diode produced from 6H-type silicon carbide with the addition of nitrogen and aluminum as a luminescent center (the emission spectrum is taken from Ikeda et al., Journal of Applied Physics, supra.). The wavelength of the light emission peak of the spectrum is 460 to 480 nm, but the peak width at half height is as large as 70 to 90 nm (0.4 to 0.5 eV). Thus, the color of light emitted from this light emitting diode is not pure blue but pale blue.
The emission spectrum of FIG. 8 has been thoroughly analyzed by Ikeda et al. As shown in this figure, there are three different emission processes F, E, and M. According to the analysis, the process M is caused by light emission of donor-acceptor pairs through the recombination of a nitrogen donor and an aluminum acceptor, the process E is not well known, but may be caused by recombination associated with an aluminum impurity, and the process F is caused by recombination of free excitons.
In order to improve the brightness of conventional blue light emitting diodes produced by liquid phase epitaxy or chemical vapor deposition, the emission process M through light emission of donor-acceptor pairs has been utilized. For the light emission of donor-acceptor pairs, a nitrogen donor and an aluminum acceptor are both added to the n-type layer constituting the p-n junction. However, the distance between the nitrogen donor and the aluminum acceptor in each donor-acceptor pair, which contributes to the light emission, varies from one donor-acceptor pair to another. Accordingly, the wavelength of generated light slightly differs from one donor-acceptor pair to another, thereby enlarging the peak width of an emission spectrum obtained from the light emitting diodes. Moreover, light emission caused by the processes F and E are mixed with light emission caused by the process M, so that the peak width of the emission spectrum is further enlarged.
In the crystal growth by chemical vapor deposition, the amounts of a source gas and an impurity gas for doping, both of which are used for the crystal growth, can be controlled with high accuracy, so that the growth of silicon carbide single crystals can be accurately controlled. However, only a few blue light emitting diodes have been produced by chemical vapor deposition (e.g., S. Nishino, A. Ibaraki, H. Matsunami, and T. Tanaka, Japanese Journal of Applied Physics, Vol. 19, p. L353, 1980). This conventional blue light emitting diode is produced by silicon carbide single crystals grown at a temperature of as high as about 1800.degree. C., so that the light emitted therefrom has low brightness.
In recent years, some reports have been made on a method for growing silicon carbide single crystals by chemical vapor deposition using monosilane (SiH.sub.4) and propane (C.sub.3 H.sub.8) as a source gas, in which the crystal growth can be effected at a relatively low temperature of 1600.degree. C. or lower by the use of a silicon carbide single-crystal substrate having a growth plane with an appropriate crystal orientation (e.g., N. Kuroda, K. Shibahara, W. Yoo, S. Nishino, and H. Matsunami, Extended Abstracts of the 19th Conference on Solid State Devices and Materials, Tokyo, 1987, pp. 227-230).
With such a low temperature, however, the decomposition of propane is insufficient, so that the propane gas should be supplied in an excess amount with respect to that of the monosilane gas, which leads to low accuracy in the control of crystal growth and also causes a great possibility that unnecessary impurities will enter the growing layer. Thus, when silicon carbide single crystals grown by this conventional method are used to produce a p-n junction light emitting diode, the resultant light emitting diode cannot attain stable emission of light with high brightness.